This invention relates generally to the art of radioactivity well logging and, more particularly, to a method and an apparatus for determining the thermal neutron capture cross-section of fluids in a well bore.
It is known in the search for oil and gas to extend drill holes through the earth formations and that these holes are normally filled with fluids, which include either/or both oil and water. Additionally, it is becoming more common in enhanced recovery operations, such as secondary and tertiary recovery projects, to inject fluids into the borehole and the adjacent formations, typically in the form of a brine solution. These forms of enhanced recovery projects are commonly referred to as log-inject-log operations.
During log-inject-log operations it may be desirable to allow the detection of where the fluid is going for the first few feet of formation, where the fluid is injected, which is useful in determining residual oil saturation in the formations. One type log-inject-log procedure involves operating a pulsed neutron decay log opposite the zone or zones of interest in a well, injecting a water solution with contrasting salinity into the zone of interest, and performing a second pulsed neutron decay log. In this method, the log readings from the two surveys are used to calculate residual-oil-saturation (ROS) from the expression: ##EQU1## where .SIGMA..sub.1 =Thermal neutron absorption cross-section from the first log,
.SIGMA..sub.2 =Thermal neutron absorption cross-section from the second log, PA1 .SIGMA.w.sub.1 =Thermal neutron absorption cross-section of original formation water, and PA1 .SIGMA.w.sub.2 =Thermal neutron absorption cross-section of injected fluid.
The injection solution used is composed by combining water with a known amount of sodium-chloride to provide a salt-water solution for which the absorption cross-section can be calculated. It is not uncommon during the course of the injection process to be required to mix more than one quantity of such injection fluid and further it is not uncommon for the absorption cross-section to differ slightly from one mixture to the next mixture. One method of controlling the quality of injection fluid is to measure the absorption cross-section of each individual mixture solution to determine that the absorption cross-sections are constant throughout all the mixtures.
A related problem is encountered in that it has proven to be difficult to measure the absorption cross-section of very small fluid samples. This difficulty is based on two facts. First, the absorption cross-section of fluids is not based solely on the salinity of the fluids. Impurities, such as trace amounts of boron, and/or gadolinium can significantly alter the absorption cross-section of the fluids making measurements unreliable. A second difficulty in determining the absorption cross-section of small fluid samples is that when using a pulsed neutron source there are unique diffusion effects which will alter the neutron decay rate. These diffusion effects will cause to be measured a slightly modified decay rate which is not the true absorption cross-section of the fluid samples.
These and other disadvantages are overcome with the present invention by providing a method and an apparatus for continuously determining the salinity and the thermal neutron capture cross-section of fluids as a function of time or depth within a borehole.